The present invention relates to temperature change of a voltage controlled oscillator, especially the voltage controlled oscillator (VCO) used in a PLL circuit, and to a compensation of a temperature variation of the VCO and a frequency characteristic due to an element manufacturing variation.
One of the inventors of the present invention, Mr. Ken Fujita proposed a voltage controlled oscillator in FIG. 9 of the above copending application. The copending application was published after a priority date of the present invention. Hereinafter, the voltage controlled oscillator shown in FIG. 9 is called as “the proposed VCO”.
The proposed VCO includes a negative resistor 31, an inductor 32, an oscillation frequency setting capacitor 33 and a buffering circuit 36. The negative resistor 31, the inductor 32 and the capacitor 33 constitute a resonant circuit. The capacitor 33 is a variable capacitance type capacitor and has a voltage control terminal for varying a capacitance value. The capacitance of the capacitor 33 is varied according to the LPF voltage. The buffering circuit 36 is a buffer circuit for preventing buffering between the resonant circuit and a circuit on the output side.
The proposed VCO further includes temperature compensating capacitors 34 and 35 and device characteristic compensating capacitors 38 through 41 and a monitor circuit 37. The device characteristic compensating capacitors 38 through 41 are respectively variable capacitances for adjusting a shift in oscillation frequency band due to variations in device characteristic. The monitor circuit 37 is a circuit for monitoring an LPF voltage. The circuit compares a reference voltage corresponding to a design value of an LPF voltage necessary to output an oscillation frequency set by frequency data (division ratio N) at a desired temperature and an actual LPF voltage and outputs the result thereof as a monitor output 37.
As described above, the proposed VCO has two temperature compensating capacitors and four device characteristic compensating capacitors for temperature. The temperature compensating capacitors and the device characteristic compensating capacitors are generically called as trimming capacitors.
Since a minimum capacitance of the trimming capacitors is not the infinitesimal, when the number of trimming capacitors increases, two or more floating capacity unrelated to a trimming signal (a digital signal) will be connected in parallel with LC resonator. In case the total sum value of the floating capacity cannot disregard compared with the variable range of the variable capacitor, when performing frequency control with sufficient accuracy, a problem arises.
For example, when an oscillation frequency is 2.5 GHz, element constants of the LC circuit are 2 nH and about 2 pF, respectively, and the floating capacity value of the trimming capacitor is about 1 pF. However, if the oscillation frequency is set to about 5 GHz, the element constants of the LC circuit should be set to 1 nH and about 1 pF, respectively, and the floating capacitance of the trimming capacitor should become the same degree of the variable range of the variable capacitor. Therefore, in case the oscillation frequency becomes high, a problem arises by including many trimming capacitors like before.
FIG. 2 shows frequency curves that could be switched by changing the capacitance of the trimming capacitor. For example, where a frequency range required for the oscillation circuit sets to 4800 to 5000 MHz, a first frequency curve 31 can be chosen between 4800 and 4860 MHz, a second frequency curve 32 can be chosen between 4860 and 4920 MHz, and a third frequency curve 33 can be chosen above 4920 MHz. By the above selection function, frequency sensitivity can be controlled to about 100 MHz/V, and a phase noise of the oscillator is also controlled. In many LC resonators with high oscillation frequency, the trimming capacitor connected to the LC resonator is used for the change of the frequency curves.
It is not easy to switch the frequency curves with sufficient reproducibility to desired oscillation frequency. Because, there are many change factors, such as a variation in the floating capacitance and a rate of amplification of many elements which constitute the oscillator, and a temperature change of the sauce/drain capacitance of the negative resistance. Especially, the influence of the floating capacitance of the trimming capacitor is significant.